1. Field of the Invention
This invention relates to a numerically controlled grinding machine permitting the so-called gap eliminating function even in a machining operation feeding a grinding wheel on the basis of a relative feed amount (relative displacement) in each feed mode without using a sizing device.
2. Description of the Prior Art
When classified in terms of the feed speed of the grinding wheel, the conventional numerically controlled grinding machines have a number of feed modes such as "fast forward," "non-grinding," "rough grinding," "fine grinding," and "fast back." From the standpoint of machining accuracy, it is desirable to contact a workpiece during operation in the mode of rough grinding, and therefore it has been the general practice to drop the feed speed to that of the rough grinding mode long before the time point of the contact with a workpiece. Consequently, this method has a drawback that the machining cycle becomes lengthy. In order to overcome this drawback, it has been attempted to shorten the machining cycle by adding the so-called gap eliminating function, feeding the grinding wheel in the non-grinding mode up to a point close to a workpiece and immediately dropping the feed speed to the rough grinding mode upon detection of the contact with the workpiece.
However, the gap eliminating function essentially requires provision of a sizing device thereby to determine the timing of switching the feed speed and measuring the outer diameter of a workpiece until the ultimate finish size is obtained. In case of a workpiece of a stepped shape, it is the usual practice to machine a first machining surface of the workpiece by the use of a sizing device and to machine a second surface on the basis of relative dimensions without using a sizing device in order to enhance the machining speed.
However, in case of grinding the second machining surface by the method based on the relative feed of the grinding wheel in each feed mode without resorting to a sizing machine, it is difficult to use the gap eliminating function. This is because the feed amount for a remaining portion of the feed for the non-grinding mode is cancelled, and the feed after that is controlled according to the predetermined feed amount for each feed mode, inviting an error in the final dimensions in a degree corresponding to the cancelled feed portion.